The present invention relates to a semiconductor device having an electrically reprogrammable non-volatile memory, and a manufacturing method thereof.
A conventional semiconductor memory device is configured in such a manner that two transistors respectively formed in a P well layer and an N well layer formed in a bulk substrate with a flash memory formed therein are connected in series by one floating gate electrode. Upon an erase operation, a control gate electrode and a drain layer are grounded and a voltage of 7V is applied to a source layer to pull out electrons from the floating gate electrode by a tunnel current, thereby reducing the threshold voltage of the flash memory. Upon a write operation, the drain layer is grounded and a voltage of 5V is applied to the control gate electrode and the source layer to inject hot electrons into the floating gate electrode, thereby increasing the threshold voltage of the flash memory, whereby data stored according to the magnitude of the threshold voltage is read (refer to, for example, a patent document 1 (Japanese Unexamined Patent Publication No. 2001-229690 (paragraph 0011 in fifth page—paragraph 0021 in sixth page, and FIG. 1)).
Since, however, the flash memory is formed in the bulk substrate in the above-described related art, a source-to-drain breakdown voltage is high and electric charges can be injected into the floating gate electrode through a gate oxide film. However, a fully-depleted SOI structure type semiconductor memory device in which an SOI (Silicon On Insulator) layer made up of thin-film silicon is stacked on an embedded oxide film, is accompanied by problems that it is difficult to sufficiently ensure a source-to-drain breakdown voltage of a MOSFET (MOS Field Effect Transistor) formed in the SOI layer, and electric charges cannot be injected into a floating gate electrode through a gate oxide film, thus encountering difficulties in forming an electrically reprogrammable non-volatile memory in the semiconductor memory device of SOI structure.
Therefore, when the time required to inject the electric charges into the floating gate electrode is long, the time required to inject the electric charges at product inspections before shipment also becomes long, thereby leading to a rise in manufacturing cost.